1. Field of the Invention
This invention relates to improved chromatographic strips and methods of preparing them. The improved chromatographic strips of the invention are useful in analytical chromatography, particularly as immunochromatographic strips.
A number of materials are known for use as chromatographic strips. Generally, the material is "bibulous" or "porous," comprising a random or oriented pile of fibers such as found, for example, in cellulose, fiberglass, woven cloth, cotton, polyester, etc., and the like. The preferred material is paper, which is a random pile of cellulose fibers. The tortuous interstitial and interconnected capillaries of such a random pile of fibers create both a drive for and a resistance to a mobile phase, which, in a chromatographic system, traverses at least a portion of the strip. The mobile phase is generally a liquid medium, most usually a solvent. The transfer of the mobile phase to channels within the random pile, which channels have ever smaller diameters, provides the free energy gradient to drive the mobile phase through the medium. Dead end pores trap the mobile phase and limit its traversal of the chromatographic strip.
The chromatographic strips are normally prepared from larger sheets from which they are cut by mechanical means. The most widely used form of mechanical cutting involves a blade or wire.
Mechanical cutting of the sheet into strips results in a deformation of the edge of the strip along the cutting line. This deformation takes the form of a compression of the edges of the strip. The fibers which form the strip, when cut mechanically, are pushed closer together at the cut edges when compared to the distance between the fibers in the body of the strip. This deformation of the edges of the strip frequently results in a faster rate of traversal for the liquid medium at the edges of the strip than through the body of the strip. The fronts of components traversing the strip become concave rather than flat.
In many situations employing a chromatographic strip it is important that the shape of the front of the traversing component be flat. In analytical and preparative chromatography, it is usually preferable to have a flat front. An example of such a situation is affinity chromatography. In such a test antibodies are attached to a porous insoluble support. During migration of an antigen-containing solution on the porous support, the migration of the antigen solute is specifically delayed in comparison to the migration of the solvent and other solutes. The relative delay decreases with increasing antigen concentration. Accurate quantitations of the concentration of analyte in a sample to be analyzed requires that the position of the analyte front relative to the solvent front be measured accurately. The position of a flat front can usually be measured with greater precision and accuracy than that of a concave front, and a higher degree of accuracy is thereby obtained in a chromatographic assay. Moreover, in preparative chromatography a linear front permits more ready separation and isolation of the pure component.
2. Brief Description of the Prior Art
U.S. Pat. No. 4,168,146 describes an immunoassay employing immunochromatography with antigens followed by contacting the immunochromatograph with an aqueous solution containing labelled antibodies. An enzyme chromatographic immunoassay is described in U.S. Ser. No. 398,505, filed July 15, 1982, now U.S. Pat. No. 4,435,504.